A container contains 0.60 m NaCl; given Kf = 1.86 °C/m and i ≈ 2, what is the freezing-point depression ΔT_f?

• A. 1.23 °C
• B. 2.23 °C
• C. 4.46 °C
• D. 2.02 °C

Answer: (B)

Freezing-point depression scales with how many particles the solute produces and how concentrated the solution is. Use ΔT_f = i · K_f · m. For NaCl, i ≈ 2 because it dissociates into two ions. With a molality of 0.60 m and K_f = 1.86 °C/m, ΔT_f = 2 × 1.86 × 0.60 = 2.232 °C, about 2.23 °C. So the solution’s freezing point is lowered by roughly 2.23 °C.